Optical disc drives have been actively developed as a way to record and reproduce large volumes of data Various approaches have been taken to increase the recording density. Phase change optical disc media drives that use the ability to change the recording layer between crystalline and amorphous states are one such approach.
Phase change optical disc drives heat the recording thin film formed on the disc substrate by emitting a laser beam, thereby causing a change in the crystalline structure of the thin film to record and erase information. Amorphous marks and crystalline spaces between the marks are formed on the optical disc by emitting the laser beam at a peak power level to convert crystalline parts of the recording film to an amorphous state, or at a bias power level to convert amorphous parts to a crystalline state. Reflectance is different in the recorded marks and spaces. When a light spot is focused on the optical disc, differences in mark and space reflectance are detected as a signal, which is then decoded to read the information.
Land and groove recording techniques enable recording marks and spaces to both the land tracks of the guide grooves on the disc and the groove tracks therebetween.
Address prepits are formed also at the factory when the guide grooves are formed in the disc. These address prepits identify specific locations (addresses) on the disc, and recessed pits and lands formed at a constant interval along the tracks. Address information is recorded by controlling whether the pits are formed or not and changing the length of the pit sequence.
A conventional optical disc drive is shown in FIG. 2. Shown in FIG. 2 are the optical disc 201, semiconductor laser 202, collimator lens 203 for converting the light beam emitted from the semiconductor laser to a parallel beam, beam splitter 204, convergent lens 205 for focusing the light beam on the optical disc surface, collective lens 206 for collecting the light beam reflected and diffracted by the optical disc onto a photodetector 207, the photodetector 207 for detecting the light collected thereon by the collective lens, playback signal operator 208 for arithmetically calculating the output voltage of the photodetector, focus controller 209 for controlling the focal point of the light spot on the optical disc surface, tracking controller 210 for controlling the position of the light spot to the tracks on the optical disc, actuator 211 for moving the convergent lens, laser drive unit 212 for driving the semiconductor laser, and signal processing unit 215.
A problem with this conventional configuration is that when data is recorded to or read from an optical disc having plural data layers accessible from one side of the disc and addresses are read from prepits formed in a second data layer (a layer deeper from the disc surface than the first data layer), absorption and reflection by the first (surface) layer causes a loss of power in the beam reaching the second layer. This loss is proportional to the transmittance of light through the first layer.
Light reaching the second layer is then reflected and diffracted by the address prepits in the second layer, passes back through the first layer, and reaches the photodetector. The amount of light in the beam reaching the photodetector is proportional to the square of the first layer transmittance and the reflectance of the second layer.
If, for example, the transmittance of the first layer is 50% and the amount of light in the beam emitted from the laser and incident on the first layer is 1, the amount of light that passes the first layer, reaches the second layer and is diffracted by the second layer, then passes through the first layer again and reaches the photodetector will be 1* (0.5*0.5)−R2=0.25−R2 where R2 is the reflectance of the second layer. In an optical disc in which the optical characteristics are controlled so that the reflectance difference (ΔR) between spaces and recording marks in the first and second layers is the same, the amount of light diffracted by the prepits and returning to the photodetector is dependent upon the transmittance of the first layer and the reflectance of the second layer. If the transmittance of the first layer is low or the reflectance of the second layer is low, a difference occurs in the signal amplitude from the address prepits in the first and second layers. This can make it difficult to accurately read the address information from the prepits in the second layer.